The invention relates to a method for controlling charging of an electric energy storage system in a vehicle comprising an electric machine which is arranged for propulsion of said vehicle, said method comprising: initiating said charging upon connection of said energy system to an external power supply via a first connector element associated with said vehicle and a second connector element associated with said external power supply; and monitoring a contact resistance defined by the connection of said connector elements.
The invention also relates to an arrangement for controlling charging of an electric energy storage system in a vehicle comprising an electric machine which is arranged for propulsion of said vehicle, said arrangement comprising a vehicle control unit arranged in said vehicle for initiating charging upon connection of said energy storage system to an external power supply via a first connector element associated with said vehicle and a second connector element associated with said external power supply; said vehicle control unit being configured for monitoring a contact resistance defined by the connection of said connector elements.
The invention can be applied in heavy-duty vehicles, such as cars, trucks, buses and construction equipment. Although the invention will be described with respect to an application in the form of a bus, the invention is not restricted to this particular type of vehicle, but may be used in other vehicles.
In the field of vehicles, there is a steady increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines.
An internal combustion engine, for example in the form of a gasoline engine or a diesel engine, offers high efficiency with relatively low fuel consumption. However, environmental concerns have led to an increase in development of more environmental-friendly power sources for vehicles. In particular, it can be noted that electrically operated vehicles has emerged as a promising alternative.
Today, there exist various types of vehicle propulsion systems comprising electric machines. For example, a vehicle can be operated by means of an electric machine solely, or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as carbon monoxide and oxides of nitrogen.
The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, such as battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a set of rechargeable battery cells which, together with control circuits, form a unit which is arranged in a vehicle and which is configured for operating an electric machine. A hybrid vehicle is also often arranged so that the energy storage system is charged during braking, by means of a process known as regenerative braking.
A vehicle which is operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is sometimes referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle uses an energy storage system with rechargeable batteries or another suitable energy source which can be restored into a condition involving a full charge through a connection to an external electric power supply. The external power supply can be in the form of the common electric grid power system which can be accessed via a conventional power cord, or can be in the form of other arrangements depending on the vehicles involved and the power need for the recharging process.
During charging, a high amount of energy must be fed into the energy storage system in a relatively short time in order to optimize the vehicle's range of driving. For this reason, the actual charging of the energy storage system is suitably implemented through a process in which a control unit on the vehicle requests a charging process to be carried out by means of an external electric power supply. This is carried out after the energy storage system and the external power supply have been electrically connected by means of suitable connector elements.
The connector elements can for example be in the form of a pantograph on the vehicle and an electrically conductive overhead wire being associated with the external power supply. In this manner, the onboard energy storage system can be connected to the external power supply for the purpose of charging said energy storage system. Other types of connector elements also exist.
When charging the energy storage system, the external charger controls the charging process by feeding a charging current through the conductive wire. A problem may arise through the fact that the two connector elements may be exposed to heavy and intensive use and also environmental influence, such as rough weather conditions. The contact surfaces on the connector elements may be worn out, especially if the connectors are used frequently, i.e. if they are frequently connected and disconnected. All these factors may contribute to excessive wear of the connector elements.
If the connector elements are worn, the contact resistance defined by the contact points of the connector elements may be relatively high. This may lead to a high loss of power in the contact point of the connector elements, which in turn may cause high temperature and in some cases even fire. For reasons of safety and cost, there is obviously a desire to avoid such conditions in which there is a risk for a high power loss in the contact interface between the connector elements.
In particular, the above-mentioned scenario may occur in connection with plug-in hybrid vehicles with high power electric components (in the magnitude of 100 kW) where the power variation is big and fast and the space for components is small and wherein there is a need for well-designed platform thinking.
The patent document US 2011/030574 discloses an electric car which is supplied with electric power by means of an electric power supply. The electric car comprises a connector element in the form of a pantograph which is associated with a temperature sensor close to the contact point between the pantograph and an overhead conductor portion connector the power supply. A temperature sensor is arranged close to the pantograph and is used for measuring the temperature, which in turn can be used as an indication of the contact resistance between the overhead conductor and the pantograph.
Even though the arrangement according to US 2011/030574 is intended to detect an abnormality of the contact portion between the connector elements, there is still a need for a more reliable, accurate and useful processes and devices for detecting the power loss over two connector elements and for controlling charging of an electric energy storage system based on such a process.
Consequently, it is desirable to provide a method and an arrangement by means of which the problem with increased temperature, possible overheating and even fire, due to increased contact resistance caused by wear in the connector elements may be overcome in a reliable and useful manner.
According to a first aspect of the invention, a method is provided for controlling charging of an electric energy storage system in a vehicle comprising an electric machine which is arranged for propulsion of said vehicle, said method comprising: initiating said charging upon connection of said energy storage system to an external power supply via a first connector element associated with said vehicle and a second connector element associated with said external power supply; and monitoring a contact resistance defined by the connection of said connector elements. The method further comprises: measuring and calculating the power loss over said connector elements during said charging; and generating an error signal if said power loss is higher than a predetermined threshold value, said error signal being dependent on the magnitude of said power loss.
An advantage with the present invention is that it can be used for avoiding urgent and potentially dangerous situations involving excessive heat due to high power loss in the contact point between the connector elements. In particular, an error signal is generated in a potentially harmful situation, wherein the error signal produced is dependent on the magnitude of the power loss, so as to allow various measures to be taken as a result of the error signal being generated.
According to one embodiment, the method comprises classifying said error signal into at least two error categories depending on the magnitude of said calculated power loss. In this manner, an improvement is obtained since different types of erroneous situations can be detected depending on the magnitude of the power loss. Also, different types of corrective measures can be taken depending on the error signal.
Furthermore, according to one embodiment, the method comprises classifying said error signal into at least one of the following error categories: i) a category in which a generated error signal initiates shutting down of said charging; ii) a category in which a generated error signal initiates a decrease of the charging current of said external power supply; and iii) a category in which a generated error signal initiates a signal corresponding to an indication that at least one of said connector elements is being faulty.
In this manner, an advantage is obtained since predictions can be made related for example to the maintenance of the external power supply, the connector elements and other components which may be subject to wear and possible breakdown. This means that different types of actions and corrective measures can be taken depending on the error at hand. The values of the power loss as measured over time can be used as an estimate of the deterioration of the connector elements. In this manner, it may be possible to predict when the connector elements should be replaced or repaired.
According to an aspect, an estimation can be made, based on the error signal, of an expected remaining period of time until at least one of said connector elements needs to be repaired or replaced.
According to one embodiment, the method comprises the steps of: measuring a voltage drop over said connector elements during said charging; measuring the charging current of said external power supply; and calculating said power loss based on the magnitude of the voltage drop and the charging current. This is a reliable and effective way of providing a measurement of the power loss.
According to one embodiment, the method comprises: measuring a first voltage of said first connector means and a second voltage of said second connector means before said charging is initiated; calibrating a measurement of said voltage drop by comparing said first voltage and said second voltage; and initiating said charging by said external power source. This is a so-called pre-charging procedure which is arranged as protection of the various components being used for the charging process.
According to a further embodiment, the method comprises: measuring the temperature associated with a contact point of said connector elements; and calculating said power loss from the magnitude of said temperature. Through the use of values related to the temperature, suitably together with values of the charging current, the power loss can be calculated in a straightforward manner.
According to one embodiment, the method comprises determining said power loss as a function of time and generating said error signal depending on said function of time. This is a further way of refining the manner in which the values of the power loss can be used for predicting maintenance of the connector elements and also avoiding breakdown due to high temperature.
According to a further embodiment, an arrangement is provided for controlling charging of an electric energy storage system in a vehicle comprising an electric machine which is arranged for propulsion of said vehicle, said arrangement comprising a vehicle control unit arranged in said vehicle for initiating charging upon connection of said energy storage system to an external power supply via a first connector element associated with said vehicle and a second connector element associated with said external power supply; said vehicle control unit being configured for monitoring a contact resistance defined by the connection of said connector elements. The control unit comprises a measuring unit for determining the power loss over said connector elements during said charging, said control unit being configured for generating an error signal if said power loss is higher than a predetermined threshold value, wherein said error signal is dependent on the magnitude of said power loss.
According to one embodiment, the arrangement comprises a voltage detector for measuring the voltage drop over said connector elements during said charging; and a current detector for measuring the charging current of said external power supply. The charging control unit is configured for calculating said power loss from the magnitude of the voltage drop and the charging current.
According to one embodiment, the arrangement comprises a temperature sensor for measuring the temperature associated with a contact point of said connector elements; wherein the charging control unit is configured for calculating said power loss from the magnitude of said temperature.
According to a further aspect, the invention relates to a vehicle comprising an arrangement as mentioned above.
According to one embodiment, the invention relates to a computer program comprising program code means for performing the steps of any of the aspects of said method when said program is run on a computer.
According to one embodiment, the invention relates to a computer readable medium carrying a computer program comprising program code means for performing the steps as mentioned above when said program product is run on a computer.
According to one embodiment, the invention relates to a control unit for controlling charging of an electrical storage system in a vehicle comprising an electric machine which is arranged for propulsion of said vehicle, the control unit being configured to perform the steps of the above method.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.